Perpendicular magnetic recording (PMR) heads, combined with a double-layered recording medium, have made it possible to extend the ongoing increase of the recording density in hard disk drives (HDD) beyond 100 Gb/in2. However as the track width shrinks, the write field decreases due to the small pole area and pole tip saturation if head-media spacing reduction cannot be suitably applied. This situation makes it difficult to achieve 1 Tb/in2. In addition, a small grain size for the recording media is required to achieve these higher recording densities. However, conventional solutions to this problem tend not to be compatible with thermal stability. Two anticipated obstacle to further areal density growth are lack of a proper head field and dealing with the media's super-paramagnetic limit.
Fortunately, new technology options are currently being explored that promise areal density growth beyond these limits. Thermally assisted magnetic recording (TAMR) is the most promising of these technologies. Bit patterned magnetic recording (BPMR) is too expensive and the manufacturing throughput is very slow. Microwave assisted magnetic recording (MAMR) is one of the candidates but its effect is too small to increase current recording densities. Additionally it is incompatible with high anisotropy media.
FIG. 1 shows a TAMR head configuration of the prior art. Laser diode 11 illuminates the inlet of optical waveguide 12, sending light through it to couple with plasmon generator 13. This enables light in plasmon mode to be emitted at the surface or edges of plasmon generator 13. Finally, a near field spot appears at the tip of the plasmon generator's air bearing surface (ABS). This tiny near field spot induces a very localized temperature rise in the recording media.
Since the media coercivity field decreases with increasing temperature, TAMR enables magnetic recording to be achievable in a medium whose coercivity is too large for recording at room temperature. However, precise alignment of the thermal spot and the head field is critical for TAMR recording to be successful.
FIGS. 2a-2c are schematic illustrations of three conventional plasmon generator/main-pole configurations currently in use for TAMR. Plasmon generator 13 is located on the leading side of the main-pole 21's leading edge at a distance of 10-60 nm therefrom. In this configuration, the center of the thermal spot is not directly under the main-pole. However the head field negative slope is inside of the main-pole.
FIG. 3 is a down-track profile of the head field in relation to main-pole 21, optical spot 31, and thermal spot 32. The head field Heff is defined in equation (1) below:
                              H          eff                =                              (                                          H                in                                  2                  /                  3                                            +                              H                y                                  2                  /                  3                                                      )                                3            /            2                                              (        1        )            
where Hin is the in-plane field and Hy is the perpendicular component of the head field at 17.5 nm from the ABS.
The thermal spot diameter should be less 100 nm, with 50 nm or less being preferred. Even when the spot center is located only 40 nm from the main-pole, the thermal spot's trailing edge will still be outside or, at best, barely at the main-pole edge. At this point, the recording transition takes place since the head field gradient now turns negative. As a result, even in TAMR recording, the transition quality is very poor and shows severe transition curvature in conventional TAMR heads. This is because the transition is basically defined by the media anisotropy gradient (which follows the temperature profile) rather than by the head field gradient.
FIG. 4 is a computed media-recording pattern for a conventional TAMR head at 2000 kFCI (Kilo Flux Changes per Inch) linear density. The transition shows severe curvature and the signal to nose ratio (SNR) is poor at only 5.52 dB. Because of this, the behavior of TAMR, as currently implemented, is dominated by thermal factors.
A routine search of the prior art was performed with the following references of interest being found:
Tapered poles of various shapes are described in the prior art. Some examples are: U.S. Pat. No. 7,532,433 (Kawato et al), U.S. Patent Application 2009/0207525 (Guan et al—Headway), U.S. Patent Application 2009/0116145 (Guan et al—Headway), U.S. Patent Application 2004/0233578 (Gao), and U.S. Pat. No. 7,038,881 (Ito et al). None of these show the particular features, that we will disclose below, that would render them suitable for plasmon-based TAMR.